1. Technical Field
The embodiments described herein generally relate to a lithium/fluorinated carbon (Li/CFx) battery. More particularly, the embodiment relates to a CFx composite cathode and method for reducing the initial voltage drop of a Li/CFx battery.
2. Description of the Related Prior Art
Fluorinated carbon or carbon fluoride (CFx) is well known for its stable properties and is widely used in lithium carbon monoflouride (Li/CFx) batteries as the cathode. Li/CFx batteries are known as having the highest theoretical specific capacity as compared with other commercial lithium batteries, such as lithium thionyl chloride (Li/SOCl2) batteries and lithium manganese dioxide (Li/MnO2) batteries.
The Li/CFx batteries are generally based upon the reaction CFx+xLi→C+xLiF, which indicate that the specific capacity of such batteries depends on the content of fluorine (i.e., the value x) in the formula CFx. Theoretically, a CFx cathode material having an averaged x value of 1.0 can have a specific capacity as high as 865 mAh/g. The standard electrode potential and electronic conductivity of the CFx material are highly dependent upon the value x.
It has been demonstrated that the potential and conductivity of CFx materials have a general tendency to increase when a decrease in the x value is realized. Consequently, there exists a trade-off between the cell capacity and cell performance. That is, in order to maximize the battery capacity, the battery performance is compromised and the inverse is true. Further, CFx materials having an x value approaching 1 are intrinsically non-conductive. Therefore, a high energy density Li/CFx battery that requires a high x value (i.e., values of 1 or greater) suffers very high internal resistance, which not only reduces the battery's operating voltage but also causes heat generation.
Despite the above noted superiority of the Li/CFx battery, these batteries are also known by those skilled in the art as having major disadvantages including: (1) significant voltage drop in the initial discharge period; (2) poor power capability; and (3) heat generation that accompanies with the discharge process, especially at high discharge rate. In particular, the initial voltage drop is due to the substantially low conductivity of the CFx cathode material and the following recovery of the cell's voltage in early discharge periods originates from the formation of conductive carbon as one of the final discharge products. The poor power capability is due to the slow kinetics of the cell reaction, which reflects as high cell resistance. Meanwhile, the high cell resistance results in heat generation. Since all disadvantages addressed above are related to the intrinsically low conductivity of the CFx cathode material, numerous disclosures made in the prior art are focused on the improvement of the electronic conductivity of the CFx composite cathodes. Such prior art efforts can be classified as three categories of: (1) lowering the content of fluorine in CFx via a physical or a chemical manner, which certainly accompanies a decrease in the specific capacity; (2) adding a second cathode material that has a higher discharge voltage to compensate for the initial voltage drop of the CFx cathode; and (3) adding highly conductive filler or coating the CFx cathode material with a highly conductive metal layer. These disclosures are illustrated in the following references discussed in detail below.
U.S. Pat. No. 5,667,916, to Ebel et al. discloses a cathode material mixture comprising a major portion of fluorinated carbon and a minor portion of metal-containing material. Examples of the second cathode material as the minor portion of the cathode active mixture include bismuth dioxide (Bi2O3), bismuth lead oxide (Bi2Pb2O5), copper sulfide (CuS), copper chloride (CuCl2), copper oxide (CuO), iron sulfide (FeS), iron disulfide (FeS2), molybdenum oxide (MoO3), nickel sulfide (Ni3S2), silver oxide (Ag2O), silver chloride (AgCl), copper vanadium oxide (CuV2O5), copper silver vanadium oxide (CuxAgyV2Oz) and mercury oxide (HgO), and mixtures thereof. In these examples, the second cathode material in the cathode active mixture is in a percentage ranging from 15% to 40% by weight.
U.S. Pat. No. 4,791,038, to Shia et al. discloses the chemical treatment of fluorinated carbon with an alkali metal alkyl or aryl compound, which gives a partial reduction of CFx and as a result, reduces or almost eliminates the initial voltage drop of Li/CFx batteries. However, in each of the above patents, there are extra process steps, which not only increase battery cost, but also reduce the specific capacity of CFx cathode material as a result of partial defluorination by the chemical treatment.
Despite of the numerous approaches disclosed in the related prior art, there still remains a need for an improved and practical fluorinated carbon composition that substantially reduces the initial voltage drop of the Li/CFx batteries.